This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-160615, filed Jun. 8, 1999, the entire contents of which are incorporated herein by reference.
The present invention relates to a selective removing method of carbon monoxide.
A polymer electrolyte fuel cell permits suppressing generation of a pollutant causing an air pollution, is high in its heat efficiency and, thus, is expected to be used widely as a low temperature operation type fuel cell in a motor car power source, a dispersed power source or the like. A platinum catalyst is used mainly in the electrode of the polymer electrolyte fuel cell. Since the platinum catalyst tends to be poisoned by carbon monoxide (CO), it is necessary remove CO from the fuel gas as much as possible. The fuel gas is manufactured by, for example, reforming the fuel such as methanol by means of steam reforming reaction or a partial oxidation reaction so as to produce a hydrogen gas. Then, CO formed as a by-product in the hydrogen manufacturing reaction is removed by a CO shift reaction of CO+H2Oxe2x86x92CO2 +H2.
The removal of CO by the CO shift reaction is limited under the restriction in terms of the chemical equilibrium. For example, where a hydrogen gas is manufactured from methanol, followed by applying the CO shift reaction to the hydrogen-containing reaction mixture, the resultant gaseous composition contains about 40 to 60% of H2, about 10% of CO2, about 20% of H2O, and about 0.5% of CO. It should be noted that, in order to prevent the polymer electrolyte fuel cell from being poisoned by CO, it is necessary to lower the CO concentration to 100 ppm or lower. Such being the situation, it is necessary to take measures for further lowering the CO concentration in combination with the CO shift reaction. As a particular measure, studied is a method in which an oxidizing agent of O2 is added to the gaseous mixture after the CO shift reaction, followed by bringing the resultant gaseous mixture into contact with a catalyst capable of selectively oxidizing carbon monoxide so as to selectively oxidize CO within the gaseous mixture for removing CO.
However, the temperature at which the catalyst capable of selectively oxidizing carbon monoxide is allowed to perform its function sufficiently is limited. In addition, the oxidizing reaction of carbon monoxide is an exothermic reaction. Therefore, if the temperature in the reaction atmosphere is elevated with progress of the selective oxidizing reaction, the selective oxidizing capability of the catalyst is lowered, resulting in failure to remove sufficiently carbon monoxide.
On the other hand, Japanese Patent Disclosure (Kokai) No. 6-296870 as claimed discloses a catalyst for cleaning the waste gas. It is taught that the catalyst consists of at least one metal selected from the platinum group elements, which is supported on a crystalline silicate having an X-ray diffraction pattern shown in Table A given in this prior art and represented in terms of the molar ratio of the oxide in a dehydrated state by a chemical formula (1xc2x10.8)R2O.[aM2O3.bMxe2x80x2O.cAl2O3]ySiO2, where R denotes an alkali metal ion and/or hydrogen ion, M denotes an ion of at least one element selected from the group consisting of VIII group elements, rare earth elements, Ti, V, Cr, Nb, Sb and Ga, Mxe2x80x2 denotes an ion of alkaline earth metals of Mg, Ca, Sr, Ba, 0 less than a, 0xe2x89xa6b less than 20, a+c=1, and 11 less than y less than 3000.
This prior art also teaches in columns [0008] to [0009] that the particular waste gas cleaning catalyst permits cleaning the waste gas containing NOx, CO and HC in accordance with reaction formulas (1) to (4) given below:
C3H6+3/2O2xe2x86x923CH2Oxe2x80x83xe2x80x83(1)
CH2O+O2xe2x86x92CO2+H2Oxe2x80x83xe2x80x83(2)
CH2O+2NOxe2x86x92N2+CO2+H2Oxe2x80x83xe2x80x83(3)
CO+1/2O2xe2x86x92CO2xe2x80x83xe2x80x83(4)
Reaction (1) given above represents activation of HC, reaction (2) represents combustion of HC, reaction (3) represents a denitrification, and reaction (4) represents combustion of CO.
An object of the present invention is to provide a method of selectively removing carbon monoxide that permits improving a carbon monoxide removal rate in selectively removing CO from a gaseous mixture containing H2 and CO.
According to a first aspect of the present invention, there is provided a method of selectively removing carbon monoxide, comprising the steps of:
preparing a catalyst bed capable of selectively oxidizing carbon monoxide and including a plurality of catalyst layers arranged in series and the plurality of catalyst layers differing from each other in the temperature of application and arranged in the order of the application temperature such that the catalyst layer having the lowest application temperature constitutes the upstream side of the catalyst bed; and
introducing a gaseous mixture containing H2, CO and an oxidizing agent of O2 into the catalyst bed through the upstream side of the catalyst bed and supplying an additional oxidizing agent to the gaseous mixture when the gaseous mixture is introduced into the catalyst layer having a high application temperature so as to selectively oxidize and remove CO from the gaseous mixture.
According to a second aspect of the present invention, there is provided a method of selectively removing carbon monoxide, comprising the steps of:
preparing a catalyst bed capable of selectively oxidizing carbon monoxide and including a plurality of catalyst layers arranged in series and the plurality of catalyst layers differing from each other in the temperature of application and arranged in the order of the application temperature such that the catalyst layer having the lowest application temperature constitutes the upstream side of the catalyst bed; and
introducing a gaseous mixture containing H2, CO and an oxidizing agent of O2 into the catalyst bed through the upstream side of the catalyst bed and supplying an additional oxidizing agent to the gaseous mixture when the gaseous mixture is introduced into the catalyst layer having a high application temperature so as to selectively oxidize and remove CO from the gaseous mixture;
wherein the catalyst in each of the plurality of catalyst layers capable of selectively oxidizing carbon monoxide and differing from each other in the temperature of application includes at least one kind of a carrier and an active metal supported on the carrier, the carrier being selected from the group consisting of Y-type zeolite, mordenite, A-type zeolite, xcex3-Al2O3, anatase and a crystalline silicate having the highest to the fifth highest peaks in the powder X-ray diffraction using CuKxcex1 ray in the lattice spacing of 3.65xc2x10.1 xc3x85, 3.75xc2x10.1 xc3x85, 3.85xc2x10.1 xc3x85, 10.0xc2x10.3 xc3x85, and 11.2xc2x10.3 xc3x85, respectively, and having the composition represented by formula (1) under a dehydrated state:
(1xc2x10.8)R2O.[aM2O3.bNO.cAl2O3].ySiO2xe2x80x83xe2x80x83(1)
where R denotes at least one element selected from the group consisting of an alkali metal and H, M denotes at least one element selected from the group consisting of VIII group elements, rare earth elements, Ti, V, Cr, Nb, Sb and Ga, N denotes at least one element selected from the group consisting of Mg, Ca, Sr, and Ba, and the molar ratios a, b, c and y are: 0xe2x89xa6a, 0xe2x89xa6bxe2x89xa620, a+c=1, and 11xe2x89xa6yxe2x89xa63000.
According to a third aspect of the present invention, there is provided a method of selectively removing carbon monoxide, comprising the steps of:
preparing a catalyst bed capable of selectively oxidizing carbon monoxide and including a plurality of catalyst layers arranged in series and the plurality of catalyst layers differing from each other in the temperature of application and arranged in the order of the application temperature such that the catalyst layer having the lowest application temperature constitutes the upstream side of the catalyst bed; and
introducing a gaseous mixture containing H2, CO and an oxidizing agent Of O2 into the catalyst bed through the upstream side of the catalyst bed and supplying an additional oxidizing agent to the gaseous mixture when the gaseous mixture is introduced into the catalyst layer having a high application temperature so as to selectively oxidize and remove CO from the gaseous mixture;
wherein at least one kind of the catalyst includes a carrier and an active metal supported on the carrier, the carrier being a crystalline silicate having the highest to the fifth highest peaks in the powder X-ray diffraction using CuKxcex1 ray in the lattice spacing of 3.65xc2x10.1 xc3x85, 3.75xc2x10.1 xc3x85, 3.85xc2x10.1 xc3x85, 10.0xc2x10.3 xc3x85, and 11.2xc2x10.3 xc3x85, respectively, and having the composition represented by formula (1) under a dehydrated state:
(1xc2x10.8)R2O.[aM2O3.bNO.cAl2O3].ySiO2xe2x80x83xe2x80x83(1)
where R denotes at least one element selected from the group consisting of an alkali metal and H, M denotes at least one element selected from the group consisting of VIII group elements, rare earth elements, Ti, V, Cr, Nb, Sb and Ga, N denotes at least one element selected from the group consisting of Mg, Ca, Sr, and Ba, and the molar ratios a, b, c and y are: 0xe2x89xa6a, 0xe2x89xa6bxe2x89xa620, a+c=1, and 11xe2x89xa6yxe2x89xa63000.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.